1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that improves the reliability of connection of a terminal portion.
2. Description of the Related Art
A liquid crystal display device includes a TFT substrate on which pixel electrodes, thin film transistors (TFT), and the like are formed in a matrix arrangement and a counter substrate on which a black matrix, an overcoat film, or the like is formed at locations as matched with the pixel electrodes on the TFT substrate as the counter substrate is opposite to the TFT substrate, and liquid crystals are sandwiched between the TFT substrate and the counter substrate. Images are formed by controlling the optical transmittance of liquid crystal molecules for each pixel.
Since the liquid crystal display device is flat and light-weighted, the applications of the liquid crystal display device are widely spread in various fields from large-sized display devices such as a TV set to a mobile telephone, a DSC (Digital Still Camera), and the like. On the other hand, the viewing angle characteristics cause a problem in the liquid crystal display device. A viewing angle characteristic is a phenomenon in which brightness is changed or chromaticity is changed between the case where a screen is seen from the front and the case where the screen is seen from an oblique direction. For the viewing angle characteristic, an IPS (In Plane Switching) mode exhibits excellent characteristics, in which liquid crystal molecules are operated by a horizontal electric field.
A liquid crystal display panel formed of a TFT substrate and a counter substrate is supplied with a scan signal, a picture signal, a common voltage, and so on from terminal portions formed on the TFT substrate. Lead lines connected to scanning lines, picture signal lines, common interconnections, and so on extend on the terminal portions, and these interconnections are formed of metals. The interconnections are protected by a gate insulating film, an inorganic passivation film, and so on. In the terminal portion, a contact hole is formed for connecting to a flexible wiring board or the like, and a metal film is exposed.
Since the metal film is corroded with moisture or the like, or oxidized with oxygen to be an insulator, the metal film is protected by covering the metal film with ITO (Indium Tin Oxide), for example, which is a conductive metal oxide, in order to prevent these events.
However, the resistance between ITO and the metal film is increased depending on manufacture conditions or use conditions for possibly causing conduction failure. Japanese Patent Application Laid-Open Publication No. 2004-361443 describes a configuration in which ITO is not used on a connecting portion between a terminal metal and an external terminal of a flexible wiring board or the like and an insulating film such as silicon oxide or silicon nitride is formed on the terminal metal, and a plurality of contact holes are formed on the insulating film for connection to the external terminals. In Japanese Patent Application Laid-Open Publication No. 2004-361443, a terminal portion is connected to a flexible wiring board or the like using an ACF (Anisotropic Conductive Film) by thermocompression bonding of the ACF.